GROWTH METHOD OF FE3N MATERIAL

Abstract

A kind of growth method of Fe3N, and the growth is in the MOCVD system, including following process: 1). the surface nitridation of sapphire substrate would be made; 2). pump in carrier gas N2, ammonia and organic gallium sources, and grow low temperature GaN buffer on substrate; 3). the temperature would be raised and grow the GaN supporting layer; 4). pump in FeCp2 as Fe sources, then grow Fe3N on the GaN supporting layer; the Fe3N granular films and the Fe3N single crystal films could be obtained. The invention realizes growing high quality Fe3N film. According to the problem of growing material with difficulty, the problems are solved by controlling and adjusting the conditions for the flux of organic gallium source and iron source, growth temperature, growth time, the flux of ammonia, and mole ratio of N and Ga. In the invention, the method is easy, the growth process could be controlled, and thus the growth method and the process control of growth technology have advancement.

Description

FIELD OF THE INVENTION

This invention involves growth method of magnetic material Fe_xN, especially growing Fe₃N, in particular, it is a kind of growth method of Fe₃N by the metal organic chemical vapor deposition (MOCVD).

BACKGROUND OF THE INVENTION

In the past few years, microelectronic devices based on GaN have rapid development, and the research of spin electronic devices about GaN get much attention, and that among them the research of magnetic metal GaN heterostructure is especially concerned. This is noticeable, that Fe₃N with hexagonal structure is epitaxial to GaN with hexagonal structure, and the lattice mismatch is only 1.8%, as shown in FIG. 1. It provides innate conditions for combining magnetic material Fe₃N with semiconductor material GaN organically[1, 2].

According to different components, Fe_xN material could be mainly divided to such two kinds: x<2; x≧2. When x<2, iron nitrides get face-centered cubic structure at such components, and present paramagnetism, so it is not be considered. When x≧2, there are mainly ξ-Fe₂N, ε-Fe₃N and γ-Fe₄N, which curie temperature is 9K, 575K and 761K. Fe₂N doesn't present ferromagnetism at room temperature, so there is no much practical applied value in magneto electronics and device design. However, ε-Fe₃N and γ-Fe₄N have ferromagnetism at room temperature, therefore they bring attention widely in magnetic storage and other magnetic function devices[3, 4].

The advantage of Fe₃N and Fe₄N in device design is that they both have high spin polarization. The spin polarizations of transition-metal and alloys are shown in Table 1[5], and the spin polarizations of magnetic oxides are shown in Table 2[6]. Theoretical calculation indicates that spin polarizations of Fe₃N and Fe₄N are 0.5 and 0.7 respectively[7, 8], so both of them could completely serve as injection layer of spin polarization current in application for spin-electronics devices.

TABLE 1
Spin polarizations of transition-metal and alloys
	Metal
M (metal)	Ni	Co	Fe	Ni80Fe20	Co50Fe50	Co84Fe16
P (%) (spin	33	45	44	48	51	49
polarizations)

TABLE 2
Spin polarizations of magnetic oxides
		Magnetic oxides
	M (metal)	CrO2	Fe3O4	La0.61Sr0.23MnO3
	P (%) (spin	90 ± 3.6	40	72
	polarizations)

Fe₃N has cubic structure, as shown in FIG. 1, which has been epitaxial on MgO(100) substrate by single crystal structure, and it has brought widely attention in fields of magnetic recording and magnetic tunnel junction[3, 4]. Due to the difficulties of material growth[9, 10], synthesis of Fe₃N and report of research on its properties is less, and the growth quality of films is generally not good[11].

At present, the growth methods of Fe₃N films are dc magnetron sputtering or pulsed laser deposition. For these growth technology could not control the formation of crystal lattice, thus could not provide well growth environment, and the Fe₃N films of good crystal quality would not be obtained. Yamaguchi et. al. used MBE technology[1], AlN/3C—SiC serving as interposed layer, and had extended c-axis oriented Fe₃N films to Si(111) substrate successfully. Furthermore, Gajbhiye et. al. had synthesized Fe₃N—GaN core-shell structure, and researched on its properties[2]. But for present general sapphire substrate, these methods could not form Fe₃N films on Al₂O₃ (0001) substrate.

• [1] K. Yamaguchi, T. Yui, K. Yamaki et al, J. Crys. Growth 301, 579 (2007)
• [2] N. S. Gajbhiye and S. Bhattacharyya, Nanotechnology 16, 2012 (2005)
• [3] T. Takahashi, N. Takahashi, T. Nakamura et al, Solid State Sci. 6, 97 (2004)
• [4] S. Kokado, N. Fujima, K. Harigaya et al, Phys. Stat. Sol. (c) 3, 3303 (2006)
• [5] J. S. Moodera, JMMM, 1999, 200:248
• [6] R. J. Soulou, Science 1999, 282:85
• [7] K. Yamaguchi, T. Yui, K. Yamaki et al, J. Crys. Growth 301, 579 (2007)
• [8] M. Sifkovits, H. Smolinski, S. Hellwig et al, J. Magn. Magn. Mater. 204 (1999)
• [9] R. Dubey, A. Gupta and J. C. Pivin, Phys. Rev. B 74, 214110 (2006)
• [10] S. Matar, B. Siberchicot, M. Penicaud et al, J. Phys. I France 2, 1819 (1992)
• [11] S. L. Roberson, D. Finello, A. D. Banks et al, Thin Solid Films 326, 47 (1998)

SUMMARY OF THE INVENTION

The problems that need to be solved of this invention are: in view of the application of Fe_xN material on magnetic storage and other magnetic function devices, the Fe_xN material need to be provided, especially the growth method of Fe₃N material, and Fe₃N of good quality could be obtained; The product quality could not satisfy the requirements by the present growth methods of Fe₃N, so new growth method of Fe₃N need to be studied. There are no reports of growing Fe₃N films on sapphire substrate by MOCVD. This invention provides a method of growing Fe₃N films on sapphire substrate by MOCVD successfully.

The technology project of this invention is: a kind of growth method of Fe₃N, which characteristic is growing it in the metal organic chemical vapor deposition (MOCVD), including following process:

1). The sapphire substrate would be heat treated at 1000-1100° C. and then pumping in ammonia for the surface nitridation at the same temperature;

2). Pump in carrier gas N₂, ammonia and organic gallium sources, and grow 5-50 nm low temperature GaN buffer on substrate which has been treated by process 1).

3). After growing low temperature GaN buffer, the temperature would be raised to 1000-1150° C., and then it would continue to grow 10 minutes, so the GaN supporting layer of 50 nm could be get.

4). After growing GaN supporting layer, pump in FeCp₂ as Fe sources, and the flux of Fe source is 50-200 sccm, and the GaN would grow.

In the system of MOCVD, the organic gallium sources is trimethyl gallium, the flux being 1-50 sccm, and the growth time of Fe₃N is 5-3600 s; the intensity of pressure in reaction chamber would be controlled at 0.1-2 atmospheric pressure; the flux of ammonia is controlled at 500-700 sccm, and V/III ratio which denotes mole ratio of nitrogen and gallium is 500-50000.

In the process of growing Fe₃N in process 4), the hydrogen is as carrier gas, and the intensity of pressure in reaction chamber is controlled at 0.1 atmospheric pressure, and the Fe₃N granular films with granular surface and the Fe₃N single crystal films with layered structure surface, and with the rise of growth temperature, the surface of Fe₃N films become from granular structure to layered structure.

The sample grows at 600° C., the surface of films is granular, and the height of granules is all less than 100 nm. With the rise of temperature, the size of granules which grow on the surface of sample at 750° C. become bigger, the density less, and it develop to flats; when the temperature reaches 900° C., the surface of films begins to form layered structure, and when the growth temperature reaches 1050° C., the layered structure of surface combine with each other, forming plane film surface.

This invention firstly grow a layer of GaN on sapphire substrate, then continue to grow ferromagnetic Fe₃N on GaN supporting layer by metal organic chemical vapor deposition MOCVD technology. Among them, using of GaN supporting layer, growth temperature of Fe₃N and flux of FeCp₂ are the key of obtaining good quality Fe₃N material in the invention.

The invention is a growth method of Fe₃N by MOCVD, especially though controlling the temperature of MOCVD growth technology, source flow and treatment of substrate, thus the growth method and the process control of growth technology have advancement. The invention prepares Fe₃N of high quality on sapphire substrate by MOCVD successfully, and directly extends Fe₃N of hexagonal structure to GaN of hexagonal structure, thus obtains ferromagnetic metal/GaN structure. Compare to other growth method of Fe₃N, the invention could directly obtain ferromagnetic metal/GaN heterostructure which lattice mismatch is only 1.8%. The method is easy, the growth process could be controlled, and the structure of Fe₃N could be controlled. According to the problem of growing Fe₃N with difficulty, the invention solves it by controlling and adjusting the conditions for the flux of organic gallium source and iron source, growth temperature, growth time, the flux of ammonia, and mole ratio of N and Ga. Through the whole growth process in the invention, high quality Fe₃N films are obtained via strict control of conditions for temperature and the flux ratio of reactants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Match figure of Fe₃N and c-plane GaN.

FIG. 2 is AFM of the sample in the invention.

FIG. 3 is the surface roughness of Fe₃N films growing at 750° C. and 1050° C. in the invention.

FIG. 4 is XRD spectra of the sample in the invention.

FIG. 5 is measure (M-H) curve for magnetic hysteresis hoop of the sample growing at 1050° C. in the invention.

FIG. 6 is the center amplification of curve in FIG. 5, and the detail of vertical and horizontal terms can be seen.

DETAIL DESCRIPTION OF THE INVENTION

The invention is a kind of method of growing Fe₃N films on sapphire substrate by the metal organic chemical vapor deposition MOCVD epitaxy growth system. It includes following process:

1). The sapphire substrate would be heat treated at 1000-1100° C. and then pumping in ammonia for the surface nitridation at the same temperature;

2). Pump in carrier gas N₂, ammonia and organic gallium sources, and grow 5-50 nm low temperature GaN buffer on substrate which has been treated by process 1).

3). After growing low temperature GaN buffer, the temperature would be raised to 1000-1150° C., and then it would continue to grow 10 minutes, so the GaN supporting layer of 50 nm could be get.

4). After growing GaN supporting layer, pump in FeCp₂ as Fe sources, and the flux of Fe source is 50-200 sccm, and the GaN would grow.

The organic gallium sources is trimethyl gallium, the flux being 1-50 sccm, and the growth time of Fe₃N is 5-3600 s; the flux of ammonia is controlled at 500-700 sccm, and V/III ratio which denotes mole ratio of nitrogen and gallium is 500-50000; ferrocene is controlled by the flux of sources, and the intensity of pressure in reaction chamber would be controlled at 0.1-2 atmospheric pressure; optimal selection is keeping 0.1 atmospheric pressure.

FIG. 1 Match figure of Fe₃N and c-plane GaN. It can be seen from the figure that, the lattice mismatch is only 1.8% when extend Fe₃N of hexagonal structure to GaN of hexagonal structure,

FIG. 2 is the atomic force microscopic picture of Fe₃N films growing at 600° C., 750° C., 900° C. and 1050° C. The size of picked study region is all 2×2 μm. As shown in FIG. 2, the sample which grows at 600° C., the surface composes by some extremely tiny granules, and the height of granules is all less than 100 nm. The size of granules which grow on the surface of sample at 750° C. become bigger, the density less. When the temperature reaches 900° C., the surface of sample begins to form layered structure, and when the growth temperature reaches 1050° C., the layered structure of surface combine with each other, forming film structure of good quality. FIG. 3 is the surface roughness of Fe₃N films growing at 750° C. and 1050° C. in the invention, and it can be seen that, the surface of Fe₃N films growing at high temperature is more flat.

FIG. 4 is XRD spectra of Fe₃N films growing at 600° C., 750° C., 900° C. and 1050° C. in the invention. The range of abscissa is from 38° to 48°, and the ordinate used log coordinate. Combining with the foregoing results of AFM, it can be seen that: when the growth temperature is relatively low (600° C., 750° C.), the epitaxial material on GaN is mainly granules of α-Fe single substance, thus the films of granules are obtained. However, when the growth temperature is relatively high (900° C., 1050), resultant material is mainly single crystal films of Fe₃N, and the Fe₃N sample growing at such condition already had extremely high c-axis orientation. We concluded that this was the result of two reasons. Firstly, the decomposition of ammonia is different at different temperature, causing difference of generating composition at different temperature. When the growth temperature is low, the decomposition of ammonia is insufficient, and only ferrocene decompose, moreover iron atom deposits on GaN film, so granules of α-Fe single substance are easy to be formed; with the rise of growth temperature, the molecules of ammonia decompose largely, react with ferrocene adequately and form Fe₃N, moreover, extend to GaN. On the other hand, this is related to properties of iron nitrides. The sample growing at high temperature (1050° C.), grows in the equilibrium process. When the temperature is high and the environment is N-rich, the Fe₃N phase of hexagonal structure could more easily exist stably. So that, for our growth of sample, when the temperature is relatively high, the resultant material is mainly Fe₃N.

FIG. 5 is the magnetic hysteresis hoop at room temperature of the Fe₃N films growing at 1050° C. in the invention. The measurement was carried out by SQUID instrument produced by Quantum Design Company, and applied magnetic field is 0-20000 Oe, as shown in FIG. 5. It can be seen that, samples all have obvious room-temperature ferromagnetism. FIG. 6 shows the center amplification graph, thus the magnetic hysteresis hoop of vertical and parallel magnetic fields could be seen more clearly.

The measurement of applying magnetic fields in parallel (H//(0001)) and perpendicularly (H⊥(0001)) to the plane of film had been made, and the result of measurement is show in FIG. 5. It can be seen that, samples all have obvious ferromagnetism at room temperature. For the two results of measurement in different applied magnetic fields' direction, when the direction of magnetic field parallel to the plane of films, magnetization intensity of sample is easier to be saturated, and when perpendicular to the plane of films, the larger applied magnetic field is need. This also indicates that, for the film sample made in the invention, the easy magnetism direction is parallel to the films, and the hard direction is perpendicular to the plane.

The invention is a kind of method of growing Fe₃N films on sapphire substrate by the metal organic chemical vapor deposition MOCVD epitaxy growth system. Fe₃N and Fe₄N have high spin polarization, and have very good prospect in the application of magnetic storage and other magnetic function devices. Fe₄N obtained great attention very early. However, owing to the difficulties of material growth, it is also a difficulty to get good quality Fe₃N. There are no reports of growing Fe₃N films on sapphire substrate by MOCVD. The invention is a method of growing Fe₃N films on sapphire substrate by MOCVD, and it is the first time in the technology.

The growth method of metal organic chemical vapor deposition MOCVD technology is commonly a kind of growth method. However, how to select substrate, how to obtain high crystal and high quality films are well worth studying. There are problems need to be solved in the production including growth technology condition, design of buffer and so on. The invention is an invention in material, an improvement in growth method, and has further development in application.

Claims

1. A growth method of Fe3N comprising: the growth of Fe3N is processed in a system of metal organic chemical vapor deposition (MOCVD) including following process:1) a sapphire substrate is heat treated at 1000-1100° C. and then ammonia is pumped into the MOCVD to process surface nitridation for the sapphire substrate at 1000-1100° C.;2) pump in carrier gas N2, ammonia and organic gallium sources, and grow 5-50 nm low temperature GaN buffer on the sapphire substrate which has been treated by process 1);3) after growing low temperature GaN buffer, temperature is raised to 1000-1150° C., and then continue to grow 10 minutes, to get a GaN supporting layer of 50 nm;4) after growing GaN supporting layer, pump in FeCp2 as Fe sources, flux of Fe source is 50-200 sccm, to grow Fe3N.

2. The growth method of Fe3N according to claim 1, wherein in the system of MOCVD, an organic gallium source is trimethyl gallium with a flux of 1-50 sccm, a growth time of Fe3N is 5-3600 s; an intensity of pressure in reaction chamber is controlled at 0.1-2 atmospheric pressure; a flux of the ammonia is controlled at 500-700 sccm, and V/III ratio which denotes mole ratio of nitrogen and gallium is 500-50000.

3. The growth method of Fe3N according to claim 2, wherein in the step 4) of growing Fe3N, hydrogen is as carrier gas, the intensity of pressure in reaction chamber is controlled at 0.1 atmospheric pressure, get Fe3N granular film with granular surface and Fe3N single crystal film with layered structure surface, and while rising of growth temperature, the surface of Fe3N films become layered structure from granular structure.

4. The growth method of Fe3N according to claim 3, wherein a sample grown at 600° C. has a films with granular surface, and heights of the granules are all less than 100 nm, while rising of temperature sizes of the granules which grow on a surface of the sample at 750° C. become bigger, density of the granules become lesser, and the granules become flat, when the temperature reaches 900° C. the surface of films begins to form a multiple of layered structures, and when the growth temperature reaches 1050° C., the multiple of layered structures of surface combine with each other, forming plane film surface.